The invention relates to techniques for simultaneously profiling both surface height variation and film thickness of a film on a substrate using phase shifting interferometry techniques.
Although optical profilers have the advantage of being noncontact instruments, they have the drawback that they record errors in height variation across boundaries of different materials if the "optical phase change on reflection" varies across such boundaries. This is in contrast to contact profilers, which have no such drawback. For test samples having areas covered by transparent films, the recorded height error is a function of film thickness, which can vary from point to point independently from the surface height variation.
There are many instances in which it is desirable to use interferometric techniques to measure the thickness of a film 3 formed on a substrate 2, as shown in FIG. 1, and simultaneously obtain the "interfacial surface profile", i.e., the profile of the interface surface 3A between the film 3 and the substrate 2 of a test sample. The closest known prior art techniques involve use of "ellipsometry", which can be used to obtain the thickness of the film. However, ellipsometry cannot be used to obtain the interfacial surface profile or outer surface profile of the test sample. Other drawbacks of ellipsometry applied to the present subject include the necessity for a very oblique angle of incidence of a test beam and a generally large sampling spot size.
It is known that when interfering beams of white light are projected onto film 3 on substrate 2, a superposition of monochromatic interference patterns is formed. Each monochromatic interference pattern is affected by the sample differently due to the wavelength-dependent phase change incurred in the light as it traverses the film. Each monochromatic interference pattern also is affected by the sample differently due to wavelength-dependent phase changes that can occur as a result of reflection at the outer surface of the film and the interface between the substrate and the film. By selecting or filtering an appropriate set of these monochromatic interference patterns from the underlying white light pattern, it is possible to transform the phases to obtain the film thickness and the interface height by means of the well-known Fresnel reflection equations.
At a conference in October, 1989 Li and Talke suggested using the difference of two independent height measurements taken at different wavelengths to eliminate surface height variation and produce a weighted difference of the phase changes on reflection at the two wavelengths. Film thickness then could be derived from this quantity. (Two-wavelength interferometric techniques are known (See commonly assigned U.S. Pat. No. 4,832,489, issued May 23, 1989 to Wyant et al.), but do not solve the above-mentioned problems for surfaces caused by transparent films or errors in height variation due to optical phase change on reflection.) However, since the data for the two wavelengths are taken at different times, it is possible for errors to arise because of slight changes in the physical location of the upper surface of film 3. The most likely source of such errors is referred to as "drift", and is due, for example, to vibration of the substrate relative to the interferometer reference surface or temperature changes in the apparatus supporting the film-substrate sample 2,3. Even minute drift errors of as little as one nanometer are significant in some applications. In addition, influences of phase changes from the reference optics and beamsplitter optics must be eliminated by a calibration step prior to measurements of test samples. However, Li and Talke suggest no specific implementation of this idea or of avoiding the effects of drift errors or the influences of phase changes from optics other than the test sample.
The problem to be solved by the present invention is how to eliminate these effects.